1. Field of the Invention
The invention generally relates to a trench MOSFET, and more particularly, to an adaptive duo-gate MOSFET.
2. Description of Related Art
Switching-mode circuit with its excellent power efficiency has been widely used in the control for power supply. In order to achieve the optimal power efficiency, an input power loss and an output power loss must be reduced. The two main factors resulting in the output power loss are conduction loss and switching loss of the output stage power MOSFET.
In recent years, trench MOSFET has been widely used because it has the characteristic of high component density per unit chip area. This characteristic may effectively decrease the turn-on resistance of the MOSFET under the same chip area. However, in order for the trench MOSFET to be used in a high voltage operation, an N-type epitaxial layer in the drain has to decrease in doping concentration with increase in thickness, therefore, the turn-on resistance of the trench MOSFET is increased.
Recently, an improved trench MOSFET known as Charge-Coupling MOSFET (CC-MOSFET) is capable of solving the aforesaid problem. This type of transistor extends the gate to the N-type epitaxial layer, so during the blocking operation in the transistor, the electric field with a two-dimensional charge balance may be generated to increase the breakdown voltage of the MOSFET. Since this structure enables an N-type epitaxial layer having higher doping concentration, which is prepared for a low breakdown voltage application, to be used in a higher breakdown voltage application, in terms for the CC-MOSFET having the higher breakdown voltage, the turn-on resistance thereof will be lower than the traditional trench MOSFET having the same breakdown voltage.
However, because the portion of the gate that extends to the N-type epitaxial layer will generate a capacitance with the surrounding oxide layer (isolation structure) and the N-type epitaxial layer, a capacitance from the gate to the drain is increased resulting in the extension in a switching time of the CC-MOSFET, and thereby leads to an increase in the switching loss.
Therefore, an improved CC-MOSFET known as split-gate MOSFET or shielded-gate MOSFET has been introduced. This type of transistor isolates the portion of the gate that extends to the N-type epitaxial layer from a portion that originally existed at a P-type well by an inter-poly dielectric (IPD) layer, so as to separate the potentials of the two portions of the gate. The portion of the gate located in the N-type epitaxial layer is electrically coupled to the potential of the source through metal interconnection for generating the two-dimensional charge balance under the blocking operation, and the high capacitance problem caused by the CC-MOSFET can be solved with the inter-poly dielectric layer, thus reducing the switching loss.
However, because the potential of the gate located in the N-type epitaxial layer is the same as that of the source, when device is operated in a conduction state, this gate, as being similar to the source, will also be at the lowest potential and thereby unable to accumulate electrons, and thus the resulting turn-on resistance will be higher than that of the CC-MOSFET.